Does /Torrents/Downloading/rtorrent.session actually exist, is it actually a directory, and is it owned and writable by the rtorrent user? If yes/yes/yes, is rtorrent already running (in other words, is there a lockfile [rtorrent.lock] in the session directory? Is this lockfile (or any other file in the session directory) owned and wirtable by the rtorrent user?

Tip: if you haven't already done so, create a separate user for rtorrent, and run rtorrent as that user -- certainly not as root! Use screen to run it in the background at all times.